1. Field of the Invention
This invention relates to electrical signal isolators for translating a signal level without establishing any conductive connection between input and output. More particularly, this invention relates to half-wave transformer-coupled isolators.
2. Description of the Prior Art
Electrical signal isolators are used in a wide variety of electrical systems, to enable signal levels to be translated from one part of a system to another without permitting the flow of direct current therebetween. Isolators are used in diverse applications such as industrial process control systems, making medical measurements of the human body, and so on.
A number of different types of isolators have been used in the past. Many isolators employ transformer coupling to establish a signal path without DC conductivity. Reference in that regard may be made to U.S. Pat. Nos. 3,946,324 (L. R. Smith), 4,054,829 (T. J. Searle), and 4,066,974 (C. J. Reinhard) simply as illustrative of different kinds of isolators; many other kinds of transformer-coupled isolators are described in still other patents and publications.
One type of transformer-coupled isolator which has significant advantages is that referred to as a half-wave isolator. In such an arrangement, a d-c (or slowly varying) input signal is modulated (chopped) by an electronic switch connected in series with the transformer primary. The switch is driven, typically by an oscillator carrier signal, so as to close the series circuit to the transformer primary on alternate half-cycles. A corresponding demodulator switch is employed in series with the transformer secondary winding to recover a signal corresponding to the original d-c signal. The demodulator switch is driven by the same oscillator signal as the modulator switch, so as to establish synchronism between the two switches.
Such half-wave signal isolators have been used extensively heretofore. However, prior isolators of that type have suffered from certain serious problems which have made such devices less than satisfactory, particularly for applications with demanding performance requirements. For example, such devices commonly are subject to undesirable drift, especially as a result of variations in ambient temperature. Also, rapidly opening and closing a switch in series with the winding of a transformer causes instability effects, such as ringing and the like, which interfere with the attainment of high performance levels.